The present invention relates to a liquid crystal display device and more particularly to a liquid crystal display device wherein liquid crystal molecules are aligned vertically (homeotropically) between two substrates when an electric field is not applied and each pixel is divided into a plurality of domains.
FIGS. 9A to 9C are cross sectional views showing a black display state, a half tone display state and a white display state, respectively, of a conventional liquid crystal display of a homeotropic alignment type. Liquid crystal material which contains liquid crystal molecules 102 having a negative dielectric constant anisotropy is filled in a space between a pair of substrates 100 and 101. A polarizer is disposed on the outer surfaces of the substrates 100 and 101, with their polarization axes being crossed at a right angle.
As shown in FIG. 9A, when a voltage is not applied, liquid crystal molecules 102 are disposed vertically to the substrates 100 and 101 to form a black display. As a voltage is applied across the substrates, the liquid crystal molecules 102 are disposed in parallel to the substrates as shown in FIG. 9C. In this case, the polarization direction of light passing through the liquid crystal layer rotates to form a white display.
As shown in FIG. 9B, as a voltage lower than the voltage for the white display state is applied, the liquid crystal molecules 102 are disposed obliquely to the substrates. Light L1 propagating in a direction perpendicular to the substrates forms a half tone color. For light L2 propagating from the lower right to upper left as viewed in FIG. 9B, the liquid crystal layer hardly exhibits the birefringence effects. Therefore, when the display screen is viewed from the upper left, it develops black. Conversely, for light L3 propagating from the lower left to upper right in FIG. 9B, the liquid crystal layer exhibits greatly the birefringence effects. Therefore, when the display screen is viewed from the upper right, it develops color near white. As above, a usual homeotropic type liquid crystal display device has poor view angle characteristics of the half tone display state.
In order to improve the view angle characteristics, a multi domain type has been proposed to divide one pixel into a plurality of domains. In a multi domain type liquid crystal display device, an inclination direction of liquid crystal molecules is aligned in one direction in each domain and is different between domains, respectively in the half tone display state. With reference to FIG. 10, examples of the structure and operation principle of a liquid crystal display device of a multi domain type homeotropic orientation (multi domain vertical alignment type (MVA type)) will be described.
FIG. 10 is a cross sectional view of a MVA type liquid crystal display device. First protrusion patterns 17 are formed on the inner surface of a glass substrate 1, and second protrusion patterns 18 are formed on the inner surface of an opposing substrate 36. The first and second protrusion patterns 17 and 18 are alternately disposed. Vertical alignment films 28 covering the protrusion patterns 17 and 18 are formed on the inner surfaces of the glass substrate 1 with TFT""s and the opposing substrate 36.
Liquid crystal material 29 which contains liquid crystal molecules 30 is filled in a space between the glass substrate 1 with TFT""s and the opposing substrate 36. The liquid crystal molecule 30 has a negative dielectric constant anisotropy. The dielectric constant of the protrusion patterns 17 and 18 is lower than that of the liquid crystal material 29. On the outer surfaces of the glass substrate 1 and opposing substrate 36, polarizer 31 and 32 are cross-Nicol disposed. When no voltage is applied, the liquid crystal molecules 30 are aligned vertically to the substrate surfaces so that a good black display state can be obtained.
As a voltage is applied across the substrates, equipotential planes indicated by broken lines 16 are obtained. Specifically, since the dielectric constant of the projection patterns 17 and 18 is lower than that of the liquid crystal layer, an equipotential plane 16 near the side walls of the projection patterns 17 and 18 inclines to be lower in the projection pattern. Therefore, liquid crystal molecules 30a near the sidewalls of the projection patterns 17 and 18 incline so as to be in parallel to the equipotential plane 16. Liquid crystal molecules 30 near the molecules 30a are influenced by the inclination of the liquid crystal molecules 30a and incline in the same direction. Therefore, the liquid crystal molecules 30 between the first and second protrusion patterns 17 and 18 are aligned with their longitudinal axes (directors) being slanted upward right. Liquid crystal molecules 30 on the left side of the first projection pattern 17 and liquid crystal molecules 30 on the right side of the second projection pattern 18 are aligned with their directors being slanted downward right.
A plurality of domains having different inclination directions of liquid crystal molecules from each other are therefore defined in one pixel. The first and second protrusion patterns 17 and 18 define the boundaries of domains. If the first and second projection patterns 17 and 18 are disposed parallel to each other, two types of domains can be formed. If the projection patterns are bent by 90xc2x0, four types of domains can be formed. As a plurality of domains are formed in one pixel, the view angle characteristics of the halftone display state can be improved.
The present inventors have found that flickers become conspicuous when a specific display state is executed for an MVA type liquid crystal display device.
It is an object of the present invention to provide an MVA type liquid crystal display device capable of suppressing the generation of flickers.
It is another object of the present invention to provide a method of manufacturing an MVA type liquid crystal display device capable of suppressing the generation of flickers.
According to one aspect of the present invention, there is provided a liquid crystal display device, comprising: first and second substrates disposed in parallel and facing each other at some distance; liquid crystal material with liquid crystal molecules having a negative dielectric constant anisotropy and filled in a space between the first and second substrates; first and second electrodes respectively formed on inner surfaces of the first and second substrates and defining pixels; a first projection pattern formed on a surface of the first electrode; a domain boundary regulating member formed on the inner surface of the second substrate, the domain boundary regulating member together with the first projection pattern regulating the boundary of each domain in which the liquid crystal molecules are tilted in a same direction; a first alignment film formed on the inner surface of the second substrate, covering the first electrode and the first projection pattern, the first alignment film vertically directing the liquid crystal molecules in the liquid crystal material; and a second alignment film formed on the inner surface of the second substrate, covering the second electrode and the domain boundary regulating member, the second alignment film vertically directing the liquid crystal molecules in the liquid crystal material, wherein at least characteristics of ones of film thicknesses, materials and surface activation degrees of the first and second alignment films are different from each other, the different characteristics are set so that a residual d.c. voltage between the first and second alignment films after an a.c. voltage is applied across the first and second electrodes becomes smaller than a residual d.c. voltage when the characteristics are a same.
The generation of flickers and burning can be suppressed by lowering a residual d.c. voltage between two alignment films.